A common method for forming parts from plastic involves the extrusion of material from an extruder. The extruded plastic, however, remains in the molten state immediately upon its passage out of the extruder As a consequence, it can suffer serious deformation between the time of its extrusion and the time that it has cooled sufficiently to become solid.
Accordingly, the molten extrudate must experience strictly controlled conditions until it solidifies. Various types of sizing or calibrating tools to accomplish this objective appear in U.S. Pat. Nos. 4,181,487 to M. Kessler and 4,411,613 to P. Gauchel et al.
A technique finding frequent use involves feeding the molten extrudate into a water bath. Devices for determining the extrudate's size or thickness and then controlling the speed at which it passes through a sizing die or sleeve and then into a cooling tank appears in U.S. Pat. Nos. 4,137,025 to K.E. Graves et al. and 4,154,563 to K.G. Johnson.
The water through which the molten extrudate travels, however, may exert a significant pressure on the extrudate before it can solidify to retain its shape. This pressure may result in the undesired deformation of the shape of the final product. In particular, this represents a likely result when the extrudate takes the form of a closed tube.
To prevent the pressure of the cooling water from deforming the molten extrudate, many cooling tanks will actually place its contents under a negative pressure. The vacuum within the sizing tank combined with the application of higher pressure inside the extrudate help maintains the shape of the latter until it can solidify. G.R. Brown et al.'s U.S. Pat. No. 4,340,340 shows a separator which finds use with a vacuum sizing tank. The separator attaches to a vacuum pump on the tank and permits the separation of liquid from gas prior to the entrance of the former into a liquid pump and the latter to a vacuum pump.
However, controlling the vacuum itself within the tank represents an important task. A variation in the actual pressure within the tank will result in a nonuniform product; a decrease in the pressure may produce a larger tube while an increase in pressure may result in the opposite effect.
Previously, two types of efforts found use to control the vacuum within the sizing tank. The first simply constituted a manual attempt to maintain the vacuum constant. In this situation, an operator simply watched a vacuum gauge. If the vacuum in the tank, as reflected on the gauge, increased, he slightly opened a valve leading into the vacuum tank. This allowed in additional external air pressure. This resulted in a decrease of the vacuum.
If the vacuum pressure became less than desired, the operator closed somewhat the valve to the external atmosphere. This increased the magnitude of the vacuum.
However, the effort of maintaining the vacuum constant through the manual adjustment of a bleeder valve displays severe limitations. First, it requires the constant presence of an operator at all times. Secondly, the constancy of the vacuum within the tank depends upon the speed of the operator's reaction while constantly watching the gauge.
A more recent effort to control the vacuum within the sizing tank involved the use of a bleeder valve automatically controlled by weights. Changing the amount of weights employed results in the varying of the vacuum achieved within the tank.
This technique represents a mechanical effort to solve the problem. However, the bleeder valve itself can become clogged because of the liquid environment in which it exists, dirt, or the like. Furthermore, it requires the keeping of weights of various magnitudes in a clean, stable, and available condition so that they may find use in particular applications. As a result, the search continues for a reliable technique to maintain the vacuum within the sizing tank at a known, constant value.